In recent years, surface mounted piezo-electric transducer units which can be easily mounted on a surface of a printed-circuit board are used for piezo-electric transducers such as crystal oscillators for clocking sources of personal computers, communication devices, and the like. In these surface mounted piezo-electric transducers, the following attempted improvements are publicly known:
(1) improving the oscillation characteristic of a piezo-electric transducer corresponding to a change in temperature by changing double and support into cantilever-like support or reducing stress caused by the difference in thermal expansion between the transducer and its container. PA1 (2) improving piezo-electric transducer production efficiency by forming on its longer sides two pairs of electrodes for connecting it to a container and thus enabling the transducer and container to be assembled regardless of the directions of their longer sides when the transducer is automatically fitted into the container.
A conventional piezo-electric transducer unit will be described below with reference to the drawings.
FIG. 1 is a perspective view of components of a conventional piezo-electric transducer unit. A piezo-electric transducer unit 100 comprises a substrate 101 of ceramic insulating materials, a thickness-mode crystal oscillator 102 (hereinafter referred to as "AT crystal oscillator 102"), and a ceramic cap 103. On an upper surface of the substrate 101, there are a rectangular concavity 104 for housing the AT crystal oscillator 102 and separating grooves 105 and 106 for separating conductive adhesives. Each of the separating grooves 105 and 106 is on opposite sides of a centerline of the substrate 101 parallel to its shorter sides. The AT crystal oscillator 102 is of the so-called convex type and has an excitation electrode 121 and outgoing electrodes 122 and 123 connected thereto, which are mounted fast on its upper and lower surfaces by vacuum evaporation, sputtering, and so forth. The AT crystal oscillator 102 may be a rectangular AT plate of uniform thickness. The AT crystal oscillator 102 is housed in the concavity 104 on the substrate 101, and the outgoing electrodes 122 and 123 are fixed, with conductive adhesives, onto connecting electrodes 107 and 110 or 108 and 109 respectively. Then frequency adjustment is performed as necessary and the cap 103 is joined onto the substrate 101, which completes the crystal oscillator unit 100.
FIGS. 2, 3, and 4 are diagrams showing the state of metallic wire on the substrate 101. FIG. 2 is a front view, FIG. 3 is a view from the direction of an arrow 130 in FIG. 2, and FIG. 4 is a rear view.
A surface (hereinafter referred to as "the mounting surface") of the substrate 101 shown in FIG. 2 is where the AT crystal oscillator 102 is mounted, and is nearly square. For convenience of the following explanation, it is assumed that a centerline of the near square parallel to its longer sides is an X-axis and that a centerline of the near square parallel to its shorter sides is a Y-axis. The concavity 104 is also nearly square and its center lines are identical to the above X- and Y-axis respectively. There are connecting electrodes at positions contiguous to four corners of the concavity 104. An X-Y plane formed by the above X- and Y-axis has four quadrants each of which has one connecting electrode. That is, the first quadrant has a first connecting electrode 107; the second quadrant has a second connecting electrode 108; the third quadrant has a third connecting electrode 109; and the fourth quadrant has a fourth connecting electrode 110.
The separating groove 105 lies both on the first quadrant and on the fourth quadrant, and is arranged between the first connecting electrode 107 and fourth connecting electrode 110 for preventing these electrodes from short-circuiting. The separating groove 106 lies both on the second quadrant and on the third quadrant, and is arranged between the second connecting electrode 108 and third connecting electrode 109 for preventing these electrodes from short-circuiting.
On the reverse of the substrate 101, as shown in FIG. 4, there are a first external electrode 111, a second external electrode 112, a third external electrode 113, and a fourth external electrode 114, the locations of which correspond to the first, second, third, and fourth quadrant on the upper surface of the above substrate 101 respectively.
The first connecting electrode 107 and second connecting electrode 108 are connected by a first metallic wire 115 and the first metallic wire 115 is connected to the second external electrode 112 by a second metallic wire 116. The second metallic wire 116 reaches from the mounting surface of the substrate 101 to the second external electrode 112 on the reverse through a side of the substrate 101 corresponding to the longer side of the mounting surface. The second connecting electrode 108 is connected to the third external electrode 113 by a third metallic wire 117. The third metallic wire 117 extends from the second quadrant having the second connecting electrode 108 to the third quadrant and reaches the third external electrode 113 through a side of the substrate 101 by bypassing the third connecting electrode 109. As described above, the first and second connecting electrodes 107 and 108 and the second and third external electrodes 112 and 113 are connected by the first, second, and third metallic wires 115, 116, and 117, which form a first group of the wiring pattern.
The third connecting electrode 109 and fourth connecting electrode 110 are connected by a fourth metallic wire 118 and the fourth metallic wire 118 is connected to the fourth external electrode 114 by a fifth metallic wire 119. The fifth metallic wire 119 reaches from the mounting surface of the substrate 101 to the fourth external electrode 114 on the reverse through a side of the substrate 101 corresponding to the longer side of the mounting surface. The fourth connecting electrode 110 is connected to the first external electrode 111 by a sixth metallic wire 120. The sixth metallic wire 120 extends from the fourth quadrant having the fourth connecting electrode 110 to the first quadrant and reaches the first external electrode 111 through a side of the substrate 101 by bypassing the first connecting electrode 107. As described above, the third and fourth connecting electrodes 109 and 110 and the fourth and first external electrodes 114 and 111 are connected by the fourth, fifth, and sixth metallic wires 118, 119, and 120, which form a second group of the wiring pattern.
In this conventional piezo-electric transducer unit, metallic wire is arranged close on the upper surface of the substrate 101. As a result a segment m.sub.0 having narrow space d.sub.0 between metallic wires belonging to the different groups becomes larger, as shown in FIG. 2. Metallic wires belonging to the different groups differ in electric polarity, and so large segment m.sub.0 having narrow space d.sub.0 between metallic wires will generate stray capacitance there. Furthermore, if the external electrodes and metallic wires arranged on the obverse and reverse of the substrate 101 belong to the different groups, stray capacitance will generate there. This stray capacitance increases equivalent parallel capacitance of the above piezo-electric transducer unit.
The equivalent parallel capacitance will be described according to equivalent circuit diagrams of a piezo-electric transducer unit of FIGS. 5 and 6. The equivalent circuit shown in FIG. 5 comprises a parallel circuit of a series circuit having an inductance L, resonance resistance value R, and capacitance value C.sub.1, capacitance value Cox of the piezo-electric transducer, and the above stray capacitance value Cop between wires. In the equivalent circuit of FIG. 6, the capacitance Cox of the piezo-electric transducer and stray capacitance Cop between wires connected in parallel in the equivalent circuit of FIG. 5 are combined into one parallel capacitance value Co. The parallel capacitance value Co is given by: EQU Co=Cox+Cop (1)
As described above, a stray capacitance value Cop of the conventional piezo-electric transducer unit is great. Therefore, there was the problem of generating great parallel capacitance Co. When this piezo-electric transducer unit is used in an oscillation circuit, great parallel capacitance may produce the problem of delay in an oscillation start or unstable oscillation.